The present invention relates to a water heater with reduced nitrogen oxides output.
Most water heaters adopt a so-called Bunsen combustion system which carries out partially premixed combustion at the primary air ratio of 0.1-0.7 which is followed by combustion with secondary air. In cases where this method is used, the amount of generated nitrogen oxides (hereinafter called NO.sub.x) is approximately 100-150 ppm when corrected to 0% of O.sub.2 (the same correction is hereinafter applied). In such a case, known examples for reducing NO.sub.x generated include a fully premixed combustion system, a flame cooling system using a radiation rod and an exhaust gas recycling system.
Although the fully premixed combustion system is capable of reducing NO.sub.x to less than 60 ppm by increasing an excess air ratio and consequently lowering the temperature of the flame, a problem exists in that it is necessary to precisely control the excess air ratio and prevent oscillating combustion and backfiring, which tends to occur in the method. This causes the manufacturers to incur a large cost increase due to their efforts to implement countermeasures.
The flame cooling method using a radiation rod calls for inserting a radiation rod in the flame so that the rod is heated until it glows red and emits radiation heat, thereby reducing the temperature of flame and, thus, the generation of NO.sub.x. Output of carbon monoxide (hereinafter called CO) is prevented by heating of the radiation rod. For this reason, however, it is necessary to use ceramics or heat resistant steel for the radiation rod In addition to the fact that such materials are costly and their durability is insufficient, there is a limit to which the radiation is able to lower the temperature and reduce NO.sub.x with this method. And, it is very difficult to reduce NO.sub.x by more than about 30% without CO emission.
As for the exhaust gas recycling method, it is widely known that the amount of NO.sub.x emission can be reduced to a half when the recycling ratio of exhaust gas is 10-15%. For the capacity of Bunsen burners of conventional water heaters, however, it is impossible to recycle exhaust gas at such a high recycling ratio as 10-15%; this may otherwise result in a lifted flame. Therefore, under the present conditions, such conventional water heaters are able to reduce NO.sub.x only to approximately 90 ppm by this method. Furthermore, it is necessary to take measures to prevent exhaust gas from causing corrosion in conduits used for recycling exhaust gas, fans and burner units; such extra efforts result in cost increases. Because of the above reasons, there is strong demand for development of a water heater which is capable of greatly reducing NO.sub.x and restricting an output of CO while maintaining a relatively simple construction and avoiding cost increases to the greatest extent possible.